Roof pitch is measured "rise over run". This is measured by taking a level and placing it at any point on the slope of the roof. Now take a measuring tape (or scale) and measure straight down to the roof at the 12" mark on the level.
The standard "run" is 12" therefore it always some number over 12.
In this case the rise over run is 4/12 - the roof rises 4" over 12".